In modern aircrafts the actuation system for the trailing edge flaps on the wings comprises a central drive unit or power control unit (PCU) with at least one drive motor, the unit being arranged in the fuselage in the center between the wings. Further, an assembly of shafts and gear boxes is arranged along the wing so as to transfer rotary power to drive stations also arranged along the wing at which stations the rotary power is converted into a translational movement of the flaps.
The output of the PCU is configured such that during operation the rotational speed at the output as well as of the shafts in the actuation assembly is rather high. In the drive stations transmission assemblies are employed which reduce the rotational speed so as to obtain the required torque at the output of the drive stations for adjusting the position of the flaps.
The advantage of initially using high rotational speeds is that the shafts and the gear boxes for transmitting the rotary power from the PCU to the drive stations do not need to be designed very rigid as the torque being transmitted is rather low which in turn does not require heavy shaft and gear box arrangements so that the entire weight of the actuation assembly is kept low.
In the afore-mentioned actuation assemblies it may occur that a transmission shaft between the output of the PCU and the drive stations breaks. This has the effect that drive stations beyond the shaft which is broken, are no longer supplied with rotary power. In order to detect such a failure, it is known from the prior art to provide a sensor at the right hand and left hand distal end of the entire shaft and gear box assembly remote from the PCU, the sensors being capable of detecting the angular position of the shafts and thus the flap position. If the sensors differs in the position more than a certain threshold this indicates a failure in the actuation system.
However, it is also possible that the transmission assembly in a drive station breaks so that this drive station no longer contributes any force for adjusting the position the flap connected to this drive station. In addition, it is also conceivable that a part of a strut assembly coupling the output of a drive station with the respective flap to move the latter, breaks. Also in this case the respective drive station can no longer contribute any force to adjust or maintain the position of the flap. In order to monitor whether one of the latter two types of failures occurred further prior art detection methods are known.
A first method is the so-called flap skew measurement. Here, position sensors are provided at each hinge point at which a flap is pivotably supported on the wing, and a difference in the position may indicate a failure in the actuation system and in particular in the part between the gear box assembly and the flap itself, namely in the drive station and in the strut assembly. However, the flaps may have a sufficient level of stiffness such that the loads applied on the flaps in flight do not lead to a deformation of the flaps that is large enough to be detected by the sensors. Furthermore, when the aircraft is on ground, significant forces may not act on the flaps such that the afore-mentioned stiffness prevents a difference in the detected positions at the hinge points, and this method would therefore not be capable of detecting failures when the aircraft is on ground.
In a further method load sensors are arranged on the struts connecting the output of a drive station and the respective flap such that the load applied to the struts can be measured during holding and positioning of the flap. Furthermore, the loads are measured both in-flight and on ground. The difference between these values is determined and compared with a predetermined threshold value. Based on this comparison it can be decided whether a failure is present or not. However, this method requires complex sensor assemblies.